2.1. Introduction
There is an ongoing need in medicine for methods and apparatuses that can accurately detect and characterize medical pathologies within tissues and internal organs in a human or animal body. There is currently a particular need for a versatile method and apparatus to provide safe, reliable, and low-cost analysis of such pathologies, which may occur in a wide variety of systems. Examples of such medical pathologies include cancerous tumors and lesions, bone fractures, and diseased tissue in the cardiopulmonary or neurological systems. Presently, the identification of such medical pathologies is accomplished primarily through the use of imaging techniques such as x-ray technology, mammography, and various cross-sectional techniques, which include computed tomography (CT), magnetic resonance imaging (MRI), ultrasound (US), nuclear medicine [i.e., single-photon emission computed tomography (SPECT)], and positron-emission tomography (PET).
Each of these existing technologies has drawbacks, however. For example, x-rays, mammography, and CT scans all use ionizing radiation and therefore present certain health risks to a patient, such as cell mutations. Also, both CT scans and MRI involve procedures that are relatively expensive, thereby hampering their widespread use. MRI in particular requires the expertise of highly trained personnel for extended periods of time to operate the devices and to interpret the results. Mammography is particularly uncomfortable for the patient since it requires that the breast being studied be compressed to allow more uniform tissue density, better x-ray penetration, and tissue stabilization. More importantly, these methods rely principally on two-dimensional images; three-dimensional data are compressed in one direction, thereby disguising three-dimensional structure information that can be critical for accurate diagnosis. Additionally, with the exception of SPECT and PET, imaging of tissue generally involves only an anatomic and gross morphologic assessment of general architecture and composition. The cost and availability of functional imaging techniques such as SPECT and PET has yet to be sufficiently reliably documented to justify their application in routine procedures. The lack of functional, or tissue characterization, imaging makes standard imaging non-specific. Lack of specificity causes further testing and increases costs.
As a result of these drawbacks, the medical community continues to explore alternative imaging and diagnostic techniques that can improve safety and reliability, and reduce cost. One area that has received limited attention involves the measurement of the electromagnetic field of living organisms; methods and devices that use such measurements are relatively crude and exploit only a portion of the data that can be collected. The invention described in this application is directed to a sophisticated method and apparatus that uses any information available from bioelectrical impedance measurements for the diagnosis of medical pathologies.
2.2. Basis of Cell Membrane Electrical Potential
An understanding of the electrical characteristics of organic tissue begins at the cellular level. Cell membranes are semipermeable lipid-protein bilayers that behave as leaky electrical capacitors. A typical cell membrane is approximately 7 nm thick and has ions distributed asymmetrically across it. The ionic gradients resulting from this asymmetric distribution are maintained in living cells by ionic pumps. Because ions naturally tend to diffuse from a higher concentration to a lower one, the concentration gradient across the cell membrane results in an electrical potential Vm. In a typical cell, this electrical potential is about −70 mV so that the strength of the cell's electrical field is substantial, of the order of 1 MV/m. This large electrical field affects the transport of charged particles through the cell membrane, and any physiological change that affects the ionic transport and permeability characteristics of the cell membrane can thus alter this electrical field in a measurable way.
The predominant intracellular cation is K+. If a cell membrane is permeable to K+ but not to Cl−, then positive K+ ions will diffuse down its concentration gradient across the cell membrane, resulting in an overall positive electric charge on the outside of the cell membrane and a negative charge inside the cell. This will continue until the Nernst potential difference of about 61 mV is reached, the Nernst potential defining an equilibrium point at which further diffusion of K+ out of the cell is opposed by the presence of the internal negative electrical charge. In contrast, the predominant extracellular cation is Na+, which will tend to flow down its concentration gradient into the cell. The Na+/K+ ATPase pump maintains the chemical gradient by pumping Na+ out of the cell and pumping K+into the cell, against their respective gradients. Without these gradients, and the pumps to maintain them, there would be no cell membrane electrical potential. There may also be a direct contribution of other electrogenic pumps to the cell membrane potential, such as the Ca2+ or the H30  pumps, which function in a similar manner.
One change in the electromagnetic character of the cell results from depolarization of the cell membrane, wherein the strength of the electropotential Vm is reduced. This may occur in at least four different ways: (1) a change in the concentration of the permanent ions in the cell's cytoplasm or extracellular space; (2) a change in the permeability of the cell membrane; (3) a change in the transport of the electrogenic pumps; and (4) a redistribution of heavy ions, which may be a response for Norden-Strom's radiographic observations. All of these changes have been observed in proliferating cells, mitogenesis, and malignant transformation, making this depolarization a viable characteristic signature that can be used in diagnosis. Studies have confirmed that proliferating cells are relatively depolarized when compared to their nondividing or resting counterparts. See, for example, H. G. Sachs, P. H. Stambrook, and J. D. Ebert, Changes in membrane potential during the cell cycle, Exp. Cell Res., 83, 362 (1974), which is herein incorporated by reference. Ionic fluxes, intracellular ionic composition, and transport mechanisms associated with mitogenesis thus all change during proliferation.
2.3. Transepithelial Electrical Potential and Cancer
Epithelial cells line many solid organs if secretion and absorption are part of their function. Examples of such organs include the breast, stomach, intestines, colon, prostate, kidney, uterus, nasopharynx, esophagus, and lung, all of which absorb and secrete various ions and water. These organs are also the sites of common malignancies, and several studies have demonstrated that transepithelial depolarization is an early feature of premalignant states. See, for example, I. Zs-Nagy, G. Lustyik, V. Zs-Nagy, B. Zarandi, and C. Bertoni-Freddari, Intracellular Na+:K+ratios in human cancer cells as revealed by energy dispersive X-ray microanalysis, J. Cell Biol., 90, 769 (1981), which is herein incorporated by reference. The human breast provides a specific example: epithelial cells line the terminal ductal lobular units (TDLU) of the breast, absorbing and secreting various ions and water. The transepithelial electrical potential is negative when comparing the luminal side (apical membrane) relative to the more hyperpolarized abluminal, or bloodstream, side (basolateral membrane). The apical cell membrane is permeable to Na−, thereby allowing Na+ to enter the cell down its electrical and concentration gradient. The Na+/K+ ATPase pump then extrudes Na+ into the abluminal space across the basolateral membrane and water either flows through the cell or through tight junctions between cells.
The transepithelial electrical potential VT is thus derived from the difference of the apical VA and the basolateral VBL potentials of cells arranged in an epithelial sheet: VT=VBL−VA. This is shown pictorially in FIG. 2, where three epithelial cells are depicted. In the figure, the abluminal breast parenchyma is at the bottom, and the epithelial cells sit on the basement membrane, denoted BM. The cell membrane is divided into distinct apical and basolateral domains by the tight junctions. Gap junctions and their associated connexin proteins provide more intercellular transport communication within the membrane. The potential at the basolateral side of the cell sheet is at VBL=−100 mV while the apical cell membrane is at a potential of VA=−70 mV. This results in a net transepithelial voltage of VT=−30 mV. Water and solutes can cross the epithelium either between the cells (paracellular) or through the cell (transcellular). The overall electrical structure depicted, with a net positive charge along the apical side of the membrane forms the basis for epithelia electrical models discussed below.
While epithelia normally maintain their intracellular Na+ concentration within a narrow range, cancer cells typically exhibit cytoplasmic [Na+]/[K+] ratios that are three to five times greater than healthy epithelial cells. This is one explanation for the electrical depolarization that is observed in malignant or premalignant tissue, and may reflect the loss of Na+ or K+ gradients across the cell membrane. In addition to depolarization of the cell membrane, there may be a decrease in electrogenic Na30  transport and activation of nonelectrogenic transporters during the development of epithelial malignancies. These changes contribute to the electrical differentiation between healthy and diseased tissue. As disease states such as cancer progress, they produce local changes in vascularization, water content, and cell division rate. These and other effects of the disease further alter the ionic concentrations in the tissue, which can be measured at the skin surface and within the neoplastic tissues. Other local effects, such as distortions in biologically closed circuits, may also occur.
It is worth recognizing that such effects do not occur uniformly throughout the diseased tissue. As a tumor grows and differentiates, it may show large variations in vascularity, water content, and cell division rate, and hence large variations in its electromagnetic character depending on where examination occurs. In addition to the previously noted electropotential changes, vascularity, water content and local increase in cellularity (i.e., more membranes) changes the manner in which induced currents would travel through the tissue and tumors. Tissue conductivity and membrane electrical storage capacity will be further defined. The electromagnetic properties at the core of the tumor (which may be necrotic) are likely to be very different from the electromagnetic properties at the margins of the tumor (which more probably contain the most metabolically active cells). Also, the tumor may not respond significantly to growth factors, while the enzymes and growth factors produced may significantly affect normal cells surrounding the tumor. It is thus desirable for a complete evaluation and diagnosis of tissue to make electrical measurements at a plurality of sites at and near the diseased area.
2.4. Measurement Techniques for Bioelectrical Parameters
In order to measure the electrical properties of tissue or an organ, the baseline electrical potential can be noted or a current I may be applied. The electrical system of the tissue is governed by Ohm's law, which may be written as V=IR, where V is the difference in electric potential across the tissue system and R is the resistance of the tissue system. When the epithelium becomes malignant, the electrical potential V and the resistance R are altered, thereby affecting the electrical characteristics of the tissue system; in general, both of these electrical parameters are measurable. Without intact membranes, dead tissue would be purely resistive. Live tissue thus needs to account for the complex alteration of resistance by intact membranes. Therefore, the complex electrical impedance Z of the tissue system may be measured in relation to the tissue resistance and membrane reactance, both of which vary according to the frequency of the applied current:Z≡R+j(ωL−1/ωC).In this relationship, j is √{square root over (−1)}, L is the inductance of the tissue system, C is the capacitance of the tissue system, and ω=v/2π is the angular frequency of the applied current [I∝ejωt]. The real part of the impedance is equal to the resistance, (Z)=R, and the imaginary part of the impedance is equal to the reactance, ℑ(Z)=ωL−1/ωC≡X. It is thus apparent that when the phase of the voltage response relative to the applied current is zero, i.e. Φ=0, the impedance of the tissue is equivalent to the resistance: Z=R. When the phase shift of the current lags behind, i.e. Φ=0, the tissue response is inductive. Conversely, the tissue is more capacitive when Φ>0. Tissues thus have characteristic electrical properties according to the frequency changes of the applied current.
Most previous methods for using potentials measured at the surface of a living organism are predicated on an overly simplistic hypothesis. Such methods operate on the basis that a disease state is indicated by a negative polarity with respect to a reference voltage measured at another site on the body while normal states are indicated by positive polarity with respect to the reference voltage. This hypothesis is inconsistent with the need explicated above for a plurality of measurements to obtain a reliable diagnosis. Even in prior-art devices that use multiple electrodes, their signals have merely been averaged so that a diagnostic determination is made from the polarity of a single average signal.
One prior-art device (“the Biofield device”) that improves on this method is described in U.S. Pat. No. 5,823,957. The device described there operates by measuring skin surface electrical potentials with a plurality of electrodes that are sufficiently sensitive to detect 2-4 mV regions of depolarization. Measurements are taken concurrently from each of the electrodes but without distinct integration of any time-related phenomena. The measurements are then analyzed by calculating electric potential differentials between average readings of different electrodes. Although the data acquisition interval may be less than one minute, a ten-minute equilibration is needed in order to allow the sensitive electrodes to stabilize with skin ionic fluxes.
To detect disease states in the human breast, the Biofield device calculates electric potentials both within the involved breast and between the involved breast and contralateral breast. One disadvantage is that these measurements are affected by factors irrelevant to the diagnosis and which make such a diagnosis more uncertain. For example, the measured electrical potentials may vary according to where the patient is in her menstrual cycle, her diet, and the time of day, in addition to whether there is underlying abnormal proliferation or cancer. The placement of the electrodes is also highly dependent on the investigator or technician localizing the lesion by palpation or estimating the closest surface trajectory based on mammographic triangulation. The uncertainty for nonpalpable lesions is thus significant, and it is desirable to have a method and apparatus that are not so highly dependent on the operator's skill. More importantly, curable early stage breast cancer is frequently non-palpable. Reliable correlation with mammogram is thus greatly needed to improve mammographic screening performances.
Another prior-art device (“the TransScan device”) is described in U.S. Pat. No. 5,810,742. This device generates an impedance image from capacitance and conductance data obtained from multiple elements in a sensing probe placed over the breast. Impedance mapping is calculated from a range of frequencies from a received signal that was pulsed (at about one volt) from a cylindrical device held in the contralateral hand. The sensitivity of impedance mapping for detecting cancer is a function of the frequency, and at particular frequencies becomes characteristic of the tissue being imaged. A two-dimensional image is reconstructed from the analysis of the capacitance and conductivity measurements (see FIG. 14 of U.S. Pat. No. 5,810,742).
It is currently unknown what effect various extraneous factors such as those enumerated above have on the results produced using the TransScan impedance-measuring technique. The amplitude of capacitance and conductivity graphs are converted into relative brightness for a gray scale spectrum. Unfortunately, potentially discriminant digital data is thus converted to an analog image for operator interpretation and discrimination. Even more problematic is that there is a significant dependency on the skill of the operator to recognize and identify image artifacts during scanning. Rather than digital separation and potential elimination, artifacts from underlying ribs and other structures thus detract from an operator's evaluation. Similar to the use of real-time ultrasound readings, the reliability of the TransScan device is intimately tied with the ability of the operator to produce consistent, clinically meaningful results.
Another prior-art bioimpedance device that is intended exclusively for use in cardiac monitoring is the BioZ, marketed by CardioDynamics Int'l Corp. [http://www.bioz.com]. This device functions by using a total-thoracic approach that correlates blood volume with the inverse relationship to resistance. Two dual electrodes are placed on each side of a patient's neck and chest and an electrical signal is transmitted through the thorax. From the assumption that the most conductive path in the thorax is along the aorta, the BioZ uses changes in resistance to deduce hemodynamic parameters. Because of this limitation, the principles on which the device is based are not applicable for analysis of other systems. Furthermore, the device is not well suited to an increase in the number of potential current pathways since even such an increase would not improve the analysis.
2.5. Physical and Mathematical Modeling
To correlate the results of impedance measurements with appropriate tissue characterization, a valid physical and mathematical model of the electromagnetic character of the tissue is needed. In the prior art, the tissue has been approximated with lumped-element models. As a representation of living tissue, such models are rather crude and their actual usefulness is correspondingly limited. Although a continuum approximation was proposed for an impedance analysis of epithelium in one case [Richard J. Davies et al., Epithelial Impedance Analysis in Experimentally Induced Colon Cancer, Biophys. J., 52, 783 (1987)], the model was not actually used to analyze data. Furthermore, ionic transport through tissue has consistently been assumed to be linear in such models, and there are no studies of tissue nonlinearity as a separate characteristic signature.
The lumped-element models have a number of limitations that are either inherent or have been introduced as a result of simplifying assumptions. For example, the general strategy used in constructing a lumped-element model has been to assign a single resistance Ri to the intracellular current flow through the epithelium, and to assign a separate single resistance Re to the extracellular current flow along the epithelium together with a capacitance Cm associated with the component of the ionic current flow that does not go across the epithelium [see generally T. Morimoto et al., A Study of the Electrical Bio-impedance of tumors, J. Invest. Surg., 6, 25 (1993)]. No serious attempt has been made to account for the random orientations of the epithelia and the resulting complex current paths associated with in vivo measurements.
A further weakness with lumped-element models is that only the capacitive reactance has been considered. This simplifying assumption has been made despite a clear showing that reactance increases with frequency over a distinct range of frequencies 1-20 MHZ. This inductive reactance is significant in characterizing the electrical properties of the tissue and should properly be considered For example, for unimpaired ionic current flow, such as that associated with currents along the epithelium or in bulk fluids including saline and blood, the reactance is expected to be primarily inductive. When the reactance increases with frequency, then the system is displaying an induction response. Conversely, current flow that consists of heavy ions that do not transport across the epithelium will result in a reactance where the capacitive component dominates over the inductive component.
Hence, it is necessary that a realistic model of tissue electrical characteristics account for both the capacitive and inductive character of the reactance. Proper characterization of tissue thus requires that both the capacitive and inductive components of the reactance be measured as functions of the frequency, drive level, and relative orientation with respect to the current drive. There is thus a need for a method and apparatus for coupling such measurements with a continuum model of different tissue types to serve as an indicator of the electrical behavior of epithelium, and hence, as a sensitive and early detector of cancer.